In a torque converter comprising a lock-up clutch, control of a differential pressure at the front and rear of the lock-up clutch (lock-up differential pressure) engages/disengages the lock-up clutch. The lock-up differential pressure is gradually raised from a predetermined initial differential pressure in order to shift the torque converter from a converter state to a lock-up state. The torque converter is shifted from the converter state to the lock-up state through a slip state. The lock-up clutch is released in the converter state, slips in the slip state, and is engaged in the lock-up state.
Japanese Patent Application Laid-Open No. H11-223263 published by the Japan Patent Office in 1999 discloses control of a lock-up clutch for preventing engine stalling, in which a lock-up clutch is gradually disengaged once the vehicle speed becomes a predetermined vehicle speed during deceleration.